The greater integration and lower power consumption of semiconductor devices (in particular, silicon devices) has advanced due to miniaturization (scaling law: Moore's law), and the development of greater integration and lower power consumption has been advancing at a pace of quadrupling every three years. In recent years, MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors) having a gate length of just 20 nm and less have been achieved, but due to the increase in prices of lithographic processes (prices of apparatus and mask set prices) and the physical limits of device dimensions (operational limits and variation limits), improvements in device performance are now being sought by approaches that differ from the scaling law that has served to present.
In recent years, rewritable programmable logic devices referred to as FPGA (Field Programmable Gate Array) have been developed as an intermediate rank between gate arrays and standard cells. FPGA enable clients themselves to implement any circuit configuration after manufacture of the chip. FPGA have variable resistance elements and are arranged to enable clients themselves to electrically connect any interconnects. The use of a semiconductor device in which an FPGA of this type is mounted enables an improvement in the degree of freedom of circuits. Variable resistance elements include ReRAM (Resistance Random Access Memory) that uses transition-metal oxides or NanoBridge (NEC trademark) that uses ion-conductors.
The operation characteristics of a variable resistance element include two types: unipolar and bipolar. The resistance of a unipolar variable resistance element is changed by the applied voltage level regardless of the polarity of the applied voltage. The resistance of a bipolar variable resistance element is changed by the applied voltage level and the polarity of the applied voltage. A bipolar variable resistance element can be used in ReRAM and NanoBridge (trademark), and a unipolar variable resistance element can be used in ReRAM.
First, regarding the operation of a unipolar variable resistance element, FIGS. 1A-1D show the operation characteristics of a unipolar variable resistance element. Here, the unipolar variable resistance element is of a configuration that includes a first electrode, a second electrode, and a variable resistance element that is interposed between these two electrodes.
When a positive voltage is applied to the first electrode, the variable resistance element transitions from the OFF state to the ON state with a desired set voltage as a threshold voltage, as shown in FIG. 1A. The OFF state signifies a state in which the resistance value between the two electrodes is high (high resistance state), and the ON state signifies a state in which the resistance value between the two electrodes is low (low resistance state). The threshold voltage depends on factors such as the film thickness, the composition, and density of the variable resistance layer.
Next, when a positive voltage is again applied to the first electrode in the variable resistance element in the ON state, the variable resistance element transitions from the ON state to the OFF state at a desired threshold voltage (reset voltage), as shown in FIG. 1B. When the application of the positive voltage is further continued to the first electrode, a set voltage is attained and the variable resistance element again transitions from the OFF state to the ON state.
On the other hand, when a negative voltage is applied to the first electrode, the variable resistance element transitions from the OFF state (high resistance state) to the ON state (low resistance state) with a desired set voltage as the threshold voltage, as shown in FIG. 1C. If a negative voltage is again applied to the first electrode in the variable resistance element in the ON state, the variable resistance element transitions from the ON state to the OFF state at a desired threshold voltage (reset voltage), as shown in FIG. 1D.
The operation of this variable resistance element in FIGS. 1A-1B is thus symmetrical to the operation of FIGS. 1C-1D, and a resistance-change characteristic is exhibited that depends only on the level of the voltage and that does not depend on the application direction (polarity) of the voltage. This type of element is defined as a unipolar variable resistance element.
Next, regarding the operation of a bipolar variable resistance element, FIGS. 2A-2D show the operation characteristics of a bipolar variable resistance element. Here, for the sake of comparison, the configuration of the bipolar variable resistance element is similar to that of the unipolar variable resistance element described hereinabove, and the voltage-current characteristic is shown when the same voltage is applied to the bipolar variable resistance element as in the case of the unipolar variable resistance element.
When a positive voltage is applied to the first electrode, the variable resistance element transitions from the OFF state (high resistance state) to the ON state (low resistance state) with a desired set voltage as the threshold voltage, as shown in FIG. 2A. Next, when a positive voltage is again applied to the first electrode in the variable resistance element in the ON state, the variable resistance element exhibits an ohmic current-voltage characteristic as shown in FIG. 2B without the occurrence of a change in resistance that was seen in the unipolar variable resistance element.
On the other hand, when a negative voltage is applied to the first electrode (FIG. 2C), the variable resistance element transitions from the ON state (low resistance state) to the OFF state (high resistance state) with a desired set voltage as the threshold voltage. Next, when a positive voltage is again applied to the first electrode in the variable resistance element that is in the OFF state, the variable resistance element transitions from the OFF state to the ON state at a desired threshold voltage (set voltage) as shown in FIG. 2D.
This variable resistance element thus transitions from the OFF state to the ON state only when positive voltage is applied to the first electrode and exhibits a transition from the ON state to the OFF state only when a negative voltage is applied to the first electrode. This type of element is defined as a bipolar variable resistance element.
The electrodes that are used in a bipolar variable resistance element are defined as follows. As described in FIGS. 2A-2D, the electrode that transitions from the OFF state to the ON state when a positive voltage is applied is defined as the “first electrode” or the “active electrode.” In contrast, the electrode that transitions from the ON state to the OFF state when a positive voltage is applied is defined as the “second electrode” of the “inactive electrode.”
Based on the above-described definitions of electrodes, the modes of connection of the electrodes when two variable resistance elements are connected in series are defined as follows. When electrically interconnecting two variable resistance elements, “interconnection of electrodes of the same polarity” is defined as electrically connecting together the active electrodes or the inactive electrodes, or alternatively, unifying these two electrodes.
On the other hand, “connection of electrodes of different polarities” is defined as a mode of connecting electrodes in which the active electrode of one variable resistance element is connected to the inactive electrode of the other variable resistance element.
An example of a bipolar variable resistance element having a high potential of exhibiting the above-described characteristics is disclosed in Non-Patent Document 1. Non-Patent Document 1 discloses a switching element that uses electrochemical reactions and metal ion movement in an ion conductor (a solid that allows free movement of ions through the application of an electric field). The switching element disclosed in Non-Patent Document 1 is of a configuration having an ionic conduction layer, and a first electrode and a second electrode provided opposite each other with the ionic conduction layer interposed. Of this configuration, the first electrode serves the role of supplying metal ions to the ion-conduction layer. Metal ions are not supplied to the ion-conduction layer from the second electrode.
The operation of this switching element is next described simply. When a negative voltage is applied to the second electrode with the first electrode grounded, the metal of the first electrode becomes metal ions and dissolves into the ionic conduction layer. The metal ions in the ionic conduction layer then become metal in the ionic conduction layer and precipitate, and a metal bridge that connects the first electrode and second electrodes is formed by the precipitated metal. The switch enters the ON state through the electrical connection of the first electrode and the second electrode by the metal bridge. On the other hand, when a positive voltage is applied to the second electrode with the first electrode grounded in the above-described ON state, a portion of the metal bridge is cut, whereby the electrical connection between the first electrode and the second electrode is cut, and the switch enters the OFF state. The electrical characteristics undergo changes starting from the stage preceding the complete cutting of the electrical connection with the resistance between the first electrode and the second electrode becoming great and the inter-electrode capacitance changing, finally culminating in the cutting of the electrical connection. To alter from the above-described OFF state to the ON state, a negative voltage should again be applied to the second electrode with the first electrode grounded.
Non-Patent Document 1 discloses a configuration and operation for the case of a two-terminal switching element in which two electrodes are arranged with an ionic conduction layer interposed between them and in which the conductive state between these electrodes is controlled.
This type of switching element features smaller size and lower ON resistance than a semiconductor switch (such as a MOSFET). As a result, this switching element is believed to hold promise as a switch applicable to programmable logic devices. In addition, because the conduction state (ON or OFF) in this switching element is maintained even when the applied voltage is turned off, it can also be considered that this switching element is applied to a nonvolatile memory element. For example, taking as a basic unit a memory cell that includes one selection element such as a transistor and one switching element, these memory cells are arranged aligned in a plurality of rows in each of the vertical and horizontal directions. By means of this arrangement, any memory cell can be selected from among the plurality of memory cells by word lines and bit lines. A nonvolatile memory can then be realized that can sense the conduction state of the switching element of the selected memory cell and read from the ON or OFF state of the switching element which information, “1” or “0,” is stored.
The above-described. Non-Patent Document 1 is shown below.